Plasma processing techniques are widely used for etching or depositing thin films on, or surface modification of, workpieces such as semiconductor substrates. As the density of semiconductor devices increases, plasma processing is being increasingly utilized because it can deposit films at lower temperatures than conventional techniques, can deposit a more conformal film, and can also deposit and etch films in situ.
A typical plasma processing system includes a plasma processing chamber and a workpiece holder for positioning the workpiece in the chamber. Inlet and outlet ports introduce a reactant gas or gases into the chamber. Electrodes are used to generate the plasma in the chamber from the reactant gas or gases. One or more of the electrodes are excited by a direct current (DC) voltage source or a radio frequency (RF) voltage source, often at 13.56 MHz.
A Langmuir probe is a device used for determining a plasma's internal discharge parameters in a plasma processing system, such as plasma densities, charged-particle concentrations, and energy distribution functions. The Langmuir probe is typically a metallic electrode of cylindrical, planar, or spherical geometry, which collects current from a plasma when a voltage is applied to the probe. The probe's current collection properties, often called the probe's "current-voltage (I-V) characteristic" or the "probe characteristic", yields information on the plasma's internal discharge parameters. The probe's current-voltage characteristic is very useful for studying plasma parameters in a wide variety of situations.
In the conventional approach to Langmuir probe operation, the probe is driven by a continuous voltage sweep such as a linear sawtooth voltage. In a DC discharge the plasma potential is invariant with time, and current-voltage characteristics are relatively easy to interpret in steady-state plasmas.
There has been considerable recent interest in RF glow discharge plasmas for plasma processing for use in etching and sputtering applications. However, in an RF discharge, the plasma potential fluctuates with time. This results in the collection of time-varying current by the Langmuir probe. Collection of time-varying current causes the current-voltage characteristic to distort, thereby causing error in the internal discharge parameter calculations.
Several techniques have been devised for reducing the error caused by the RF interference. For example, in a publication by E. Eser, R. E. Ogilvie, and K. A. Taylor, J. Vac. Sci. Technol. Vol. 15, p. 199, 1978, a method of reducing the error by filtering the probe characteristic leading to a time-averaged probe characteristic is disclosed. Unfortunately, the time-averaged probe characteristic does not correspond to the true probe characteristic because of the strong nonlinearity of the probe characteristic.
Another attempt to reduce the error caused by the RF interference is described in J. D. Swift and M. J. R. Schwar, Electrical Probes for Plasma Diagnostics, 137-151 (1971). In this text, floating double probes that ride up and down with the plasma potential and obtain a true DC probe characteristic are described. However, in addition to the physical limitations of double probes, it is difficult to completely float the DC voltage supply, and RF fields distort and generate spikes in the probe characteristic.
Another attempt to reduce the error caused by the RF interference is described in S. E. Savanas and K. G. Donohue, Proc. Gaseous Elec. Conf., 41, Paper PA-4 (1988). In the Savanas et al. paper, capacitive probes that have a high impedance to ground are used to monitor the time-varying plasma potential in an RF discharge. A numerical simulation of the effect of the time-varying potential on the probe characteristic is used to estimate the extent of the RF interference and correct the probe characteristic. The corrected probe characteristic is then used to estimate the plasma parameters. Unfortunately, this numerical simulation requires one to assume a Maxwellian electron energy distribution function, an assumption that is often highly erroneous. Also, if the plasma potential fluctuation is large, the probe might operate in electron saturation during part of the cycle, which violates assumptions made in the numerical simulation.
Another attempt to reduce the error caused by the RF interference is described in a publication entitled Use of Electric Probes in Silane Radio Frequency Discharges, by E. R. Mosburg, R. C. Kerns, and J. R. Abelson, J. Appl. Phys., Vol. 54, p. 4916, 1983. In the Mosburg et al. paper, to ensure that only a pure DC bias exists between the probe and the plasma, an RF signal is applied to the probe and adjusted in amplitude and phase to match the local time-varying plasma potential. Under these conditions, the probe characteristic obtained is equivalent to one obtained in a DC discharge. Unfortunately, in addition to difficulties created by the complex experimental arrangement, mismatch between the probe and plasma potential waveforms due to differences in harmonic content causes distortion of the probe characteristic.
Yet another attempt to reduce the error caused by the RF interference is described in a publication entitled Pressure Dependance of Electron Temperature Using RF-Floated Electrostatic Probes in RF Plasmas, by A. Cantin and R. R. J. Gagne, Appl. Phys. Lett., Vol. 30, p. 316, 1977. In the Cantin et al. paper, the RF voltage across the probe-plasma sheath is reduced to small values by using two probes. One of the probes acts as the Langmuir probe, while the other functions as a high impedance voltage probe that is used to sample the instantaneous plasma potential. The sampled voltage is fed to a unity gain amplifier (a follower circuit), whose output is connected to the Langmuir probe. Again, a complex arrangement is necessary.
Yet another attempt to reduce the error caused by the RF interference is described in A Tuned Langmuir Probe For Measurements in RF Glow Discharges, by Ajit P. Paranjpe, James P. McVittie and Sidney A. Self, J. Appl. Phys., Vol. 67, p. 6718, 1990. In the Paranjpe et. al paper, the RF voltage across the sheath is minimized by ensuring that the impedance of the sheath is small when compared to the impedance between the probe and ground. This is accomplished by using a passive circuit that effectively makes the Langmuir probe behave as if it were connected to a high impedance circuit. The probe is tuned for a particular plasma condition by manually mechanically adjusting a variable capacitor that is part of a tuning network outside the probe sheath. The probe is tuned by maximizing the floating potential. Unfortunately, this system requires that the probe be very short and fixed in position.
Notwithstanding the above improvements, there is still a need in the art for an improved system for reducing the error caused by the RF interference. In particular, there is a need to provide a system which reduces detuning of the probe during operation and a simple component arrangement to obtain plasma parameters.